1. Technical Field
The present invention relates generally to semiconductor fabrication, and more particularly, to methods for reducing damage to an ultra low dielectric constant (ULK) dielectric during removal of a crosslinked polymer.
2. Related Art
During the semiconductor fabrication process, it is advantageous to use a crosslinked polymer as a planarizing layer to fill via openings for other processing. Subsequently, the crosslinked polymer must be removed. Unfortunately, crosslinked polymers are difficult to remove and require relatively longer stripping times or more aggressive plasma conditions. As a result, conventional removal procedures cause damage to a surrounding ultra low dielectric constant (ULK) dielectric. For example, damage may include residue(s) on the sidewalls of the ULK dielectric, conversion of the ULK dielectric to a more silicon dioxide-like material or removal of carbon.
FIGS. 1A-C show an illustrative conventional method for removing a crosslinked polymer. As shown in FIG. 1A, at a starting point of this stage, a structure 8 includes, for example, a metal 10 in a substrate 12, hardmask 14, cap layer 16, an ultra low dielectric constant (ULK) dielectric 18, a hydrogenated silicon oxycarbide (SICOH) hardmask layer 20 and a hard mask 22 (e.g., tetraethyl orthosilicate Si(OC2H5)4 (TEOS)). Hard mask 22 would be used as a sacrificial process-assist layer for via reactive ion etch (RIE) and strip. Structure 8 would then have a crosslinked polymer 24 applied into the via opening and baked. Crosslinked polymer 24 has been found advantageous at this stage because it exhibits good image transfer to ULK dielectric 18. Next, a barrier layer 26 would be deposited, followed by line level lithography including, typically, use of an anti-reflective coating (ARC) 28 and a patterned photoresist 30. As shown in FIG. 1B, line level RIE would remove parts of ULK dielectric 18, leaving part of crosslinked polymer 24 in a bottom of the via opening. FIG. 1C shows the next step including a removal of the crosslinked polymer 24 (FIG. 1B) from the bottom of the via opening, which would be followed, typically, by a wet clean. FIG. 1C also shows the plasma-induced damage 32. The relatively longer duration or aggressive plasma condition required to remove the crosslinked polymer 24 from the bottom of the via opening adds to damage 32.
In view of the foregoing, there is a need in the art for a solution that reduces the damage to ULK dielectric during removal of crosslinked polymer.